Plastic lenses are lightweight, highly tough and easy to be dyed. Properties particularly required for plastic lenses are: low specific gravity; high transparency; low yellowness; high refractive index and high Abbe number as optical properties; high heat resistance; high strength; and the like. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens.
Recently, many organic compounds having a sulfur atom to be used for providing a high refractive index and a high Abbe number have been reported. Among such compounds, polyepisulfide compounds having a sulfur atom are known to provide a good balance between the refractive index and the Abbe number (Patent Document 1). High refractive indexes of 1.7 or higher were achieved by optical materials obtained from polyepisulfide compounds of these inventions, but a material having a refractive index higher than those has been desired, and an optical material comprising an inorganic compound having a sulfur atom and/or a selenium atom has been proposed (e.g., Patent Document 2). With respect to optical materials obtained by introducing sulfur in a polyepisulfide compound, a method of deaerating a resin composition under specific conditions in advance to improve transparency and heat resistance has been reported (Patent Document 3).